As for why Merkel went right-populist, SZ thinks it is because of the threat to government majority in parliament by rebels who'd vote against another rescue package for Greece

How does it make sense, then, for Merkel to propagate xenophobic clichés against the supposed recipients of the "rescue"? That's just likely to strengthen the resolve of the rebels within the CDU/FDP majority as they can now expect the public to support them.

Unless Merkel is framing it as "we want to force those lazies to reform and the only way to do it is with the carrot of a rescue". Is she?

Paraphrased, Merkel didn't say 'Southerners are lazy', she said 'Southerners, stop being lazy if you want our tax money'. So no carrot there, this is the pretence of a stick (a rhetorical stick in the imaginary right-wing framing of the situation, unrelated to the actual stick). Whether this will work on the rebels, in particular on the more numerous FDP rebels (who still believe only tax cuts will boost their poll numbers and feel existential angst even more after their party dropped from yet another regional parliament), is questionable. Merkel is likely to achieve more with standard fall-in-line-or-else appeals to conservative submission to authority. Which again underlines the fact that Merkel will risk sowing the seeds of hatred in millions and make big strategic and policy sacrifices for the most insignificant of political position power gains.

I should add: in the meantime, Merkel upped the ante. Her spokesman reacted to opposition criticism by insisting that what she said wasn't campaign rhetoric but a serious proposal (at a local party event?...). And also declared that 'Everyone in Europe has to put in effort to remain globally competitive, Germans too' – so 'reforms' will continue domestically, too?